Wireless communication systems have evolved substantially over the last two decades. The explosive growth of the wireless communication market is expected to continue in the future, as the demand for all types of wireless services is increasing. Due to their ability to provide high data rates for multimedia applications, Orthogonal Frequency Division Multiplexing (OFDM) is gaining a strong interest for wide-area, local area, and personal area networks. In OFDM, carrier frequencies are chosen in such a way that there is no influence of other carriers in the detection of the information in the carrier of interest when the orthogonality of the carrier is maintained. Maintaining this orthogonality requires some special care for the frequency and symbol timing synchronizations. Cyclic extension of the symbols is usually used to relax the requirements for symbol synchronization.
In wireless OFDM systems, channel estimation is an integral part of the coherent receiver design as the performance of the receiver is greatly affected by the quality of the channel estimation. Extensive studies on the topic can be found in the literature. In OFDM systems, channel estimation is frequently employed in the frequency domain after taking the Discrete Fourier Transform (DFT) of the time synchronized digital samples. There are numerous approaches for estimating the Channel Frequency Response (CFR) over the OFDM subcarriers. The direct Least-Squares (LS) estimation assumes the channel over each subcarrier to be independent. However, in practice, the CFR is often oversampled over these subcarriers, and the estimated coefficients are correlated. On the other hand, the noise in these subcarriers can be independent. By exploiting the correlation of CFR over OFDM subcarriers, the noise can be reduced significantly, and hense the channel estimation accuracy can be improved. Assuming that the channel frequency correlation and noise variance are known, Minimum Mean-Square Error (MMSE) filtering of the LS estimates has been shown to provide optimal performance under Additive White Gaussian (AWGN).
Many of the channel estimation approaches proposed in the literature assume perfect symbol timing. However, in practice, the symbol timing used in OFDM systems is not perfect. As such, the symbol timing is often intentionally shifted towards the Cyclic Prefix (CP) so that any possible error in symbol timing that might create the loss of orthogonality can be avoided. Even though this intentional bias in synchronization avoids the loss of orthogonality of the carriers and intercarrier-interference, it results in the effective CFR to be less correlated due to the additional carrier-dependent phase shift. Synchronization errors in the receiver cause a linear phase rotation at the output of the DFT block. The correlation between the channel coefficients at different subcarriers is weakened due to this phase rotation. As a result, the performance of MMSE channel estimation degrades significantly since the noise averaging effect will be reduced.
Accordingly, what is needed in the art is an improved system and method for performing Minimum Mean-Square Error (MMSE) channel estimation in an Orthogonal Frequency Division Multiplexing (OFDM) channel under synchronization errors.